Booting problem

My mate has a Duron 800 using windows 98.He has a problem whereby the computer does not boot up.I looked at it and went into the bios and found that his hard drive was "not installed".I changed this to "user" and saved the settings when I exited the bios.It worked fine fine 5 or 6 goes,then the fault happened again.Sometimes when it is "not installed" it is possible to get to "user",other times it is not.Would I be right in suspecting his hard drive is about to pack up?

I fitted a new battery but it has not made a difference.I managed to stay on the comp for about 10 minutes and then it froze,when I rebooted it ,it came up that hard drive was "not installed".It appears that once it is warmed up I cannot access the "user" function,although it will come on ok the first time I switch on.All the fans are working and the temperatures are 30 /38 degrees.